1. Field of the Invention The invention relates to a process for the preparation of thymol by reaction of m-cresol with propene on wide and medium-pored zeolites.
Thymol is important as odourant in the perfume industry, as antiseptic and also as component of pharmaceutical products. Moreover, it is the preferred starting material for the synthesis of menthol (Ullmanns Enzyklopadie der technischen Chemie [Ullmanns Encyclopaedia of Industrial Chemistry], 4th edition, volume 20, p. 241 and volume 18, p. 213).
2. Description of the Related Art Several processes for the alkylation of m-cresol with propene have been described in the literature. In these processes, apart from thymol mainly the isomeric isopropylmethylphenols and dialkylated and polyalkylated products are obtained (Ullmann, loc. cit., volume 18, p 204). According to DE-OS (German Published Specification) 2,528,303, thymol is prepared by alkylation of mcresol with propene in the liquid phase at 360.degree.-365.degree. C. and 48-50 bar over activated aluminas. The selectivity of thymol in the crude product is 80 %. Pure thymol is recovered from this product by fractional distillation. Calcium oxide (JP 45/15491 (1970)) and aluminium m-cresolate (GB 1,227,924) have also been described as catalysts for the synthesis of thymol. However, these catalysts only display useful activity and selectivity if the reaction is carried out in autoclave reactors under superatmospheric pressure.
In DE-AS (German Published Specification) 1,815,846, aluminium oxides which are impregnated with metal sulphates are described as catalysts. In this case, too, the reaction is carried out in the liquid phase in autoclaves. To achieve a conversion of 90-99 %, a three-fold excess of propene is required. The thymol selectivity is said to be 80-82 %, although it is not specified in this publication. In a publication written by the same author (Bull. Chem. Soc. Japan 49 (1976), 2669), similar data are presented. Using the already mentioned metal sulphate catalysts, good selectivities are only achieved with a three-fold excess of propene and conversions of 5-28 %. However, not all the by-products are included in the calculation of the selectivity. Since an increased formation of by-products takes place in the case of a high propene excess, the actual selectivities are clearly below the published values.
All these known processes have in common that, in order to achieve good conversions and selectivities, the reactions have to be carried out in the liquid phase under superatmospheric pressure, since the effect of the catalysts described is unsatisfactory under the conditions of atmospheric pressure. The resulting disadvantages are a consequence of the difficulties associated with carrying out reactions under pressure in industry.
For this reason, the gas phase propylation of m-cresol under atmospheric pressure has already been investigated. Using metal sulphates on alumina or silica support materials, maximum selectivities of 70 % are achieved in combination with conversions of up to 42 % (Bull. Chem. Soc. Japan 47 (1974), 2897). In addition to the low yield, the rapid deactivation of the catalyst described is another reason against its application in industry. In Bull. Chem. Soc. Japan 47 (1974), 2360, a .gamma.-Al.sub.2 O.sub.3 is described as a catalyst for the gas phase propylation of m-cresol under atmospheric pressure. The conversion is said to be 63 % and the thymol selectivity 90 %. According to the definition given, the calculation of the selectivity in this publication only refers to the thymol isomers. Polyalkylated compounds which amount to up to 10 % of the reaction product are not included in the calculation. As a result, the actual selectivity, that is, the relative percentage of thymol, based on all the reaction products, is considerably less than the values given. This has been confirmed by in-house comparative experiments using Al.sub.2 O.sub.3 catalysts.
It was therefore desired to develop a process for the preparation of thymol from m-cresol and propene which can be carried out in the gas phase not only at atmospheric pressure but also at superatmospheric pressure and which furthermore gives good conversions and high selectivities in combination with long catalyst lives. Given the prior art, it was surprising and unforeseeable that the abovementioned disadvantages can be avoided by carrying out the reaction over the zeolite catalysts described below, which have a pore diameter of at least 5 .ANG. (wide- and medium-pored zeolites).